The invention relates to a coupling for transmitting torque between two parts which are rotatable relative to one another, which coupling comprises a friction coupling whose friction plates are each non-rotatably connected alternately to the one or to the other of the two parts which are rotatable relative to one another, having a pressure chamber which is filled with a highly viscous fluid and which is formed by a housing formed in one of the rotatable parts and by a displaceable piston sealed relative to said housing, and having a pump unit in which there rotates a conveying member connected to the other one of the rotatable parts, wherein a reservoir for a highly viscous fluid is provided in the housing, which reservoir is connected to the pump unit, and wherein the pressure chamber is supplied by the pump unit and wherein the piston loads the friction coupling.
Couplings of said type can be used as locking devices in axle differentials or central differentials, with axle differential, in the present context, meaning the conventional differential drive between the wheels of a driven vehicle axle, which differential drive is driven by a propeller shaft, and with central differential meaning the differential drive between two driven axles of a motor vehicle, which differential drive is driven by an intermediate driveshaft. In addition, couplings of said type can be used directly in a propeller shaft for a drive axle which is driven when required and which is torque-loaded only in the case of slip at a permanently driven drive axle. Couplings of said type are known, in particular, to have fluid shear pumps of the Visco-Lok.RTM. type as actuating elements. The fluid shear pumps used in said couplings, in view of the design principle, can only generate a limited volume flow which, with an increasing pressure, decreases towards zero even if there are no leakages. The volume flow of the fluid shear pumps can be increased by a wider or deeper shear channel. However, in view of the space restrictions, such an increase has its limits. The Visco-Lok.RTM. couplings provide soft connecting characteristics which are advantageous in avoiding engaging impacts and torque peaks, for example when used in pick-up vehicles and off-highway vehicles and which benefit the braking stability of the vehicle. For applications involving high demands regarding the reaction speed, the volume flow generated by the fluid shear pumps of Visco-Lok.RTM. couplings is not always sufficient.